In the Build 2011 keynote and demos, they were always talking about C++, C#, Visual Basic and JavaScript. Can't I build Metro style Apps with F#? Or is it safe to say that if it works with C#, then it works with F# as well?

It's not quite as easy. True, the runtime projection is entirely transparent to anything that compiles to IL, but most compilers also need to load type information from assemblies at compile-time (for type checking). This means being able to read and parse .winmd files, which have a few extensions compared to vanilla CLI assemblies. Furthermore .winmd files describe WinRT types as is, without applying CLR projection rules - e.g. if you read them you see a type implementing IIterable<T> WinRT interface, but at runtime CLR will map it to IEnumerable<T>. So compiler support is needed.
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Pavel MinaevSep 17 '11 at 19:10

You cannot reference ordinary .NET dll’s (even in C#). They have to
be have recompiled with a “reference” to the WinRT. Because of this
you cannot reference the F# dll. I think this will be solved when F#
3.0 get released (source: Understanding F# slide from Don Syme at
build).

The ability from F# to create Metro apps. This has to be
built by the F# team. This is an open question. This is the all-F#
approach (In WPF it is today possible but not recommended).

Tool support for creation Metro front-end projects. This will be not the case for the
F# 3.0 release. (source: Understanding F# slide from Don Syme at
build).

Using Visual Studio 11 Developer Preview (Full, not Express), I was able to reference and run F# code from a C# Metro style application with the following caveats:

You will get a build warning saying that the project can't be referenced, even though Visual Studio will still allow you to add it.

You must remove all references to all DLL's in the F# project, including FSharp.Core.

By following these steps it is possible to run F# code from a C# Metro style application, but it is nearly impossible to write any useful code in F# without references to the core language libraries. Let's hope they update the libraries to be WinRT compatible soon.

The question has been answered here:
Windows 8 and F#
Although you cannot create and design Metro apps directly in F#, you can still create so called Portable library in F# with all Models and ViewModels and use it in another Metro projects. It's not ideal, but it works.

I ported my stock chart library for .NET3.5-4.0 to Portable library,it works quite well on Metro with C# Metro application and some my Metro UI bridge.
I'm convinced that I can use F# to make Metro application in many parts.